Ohmic contacts for p-type wide bandgap II-VI semiconductor materials

Abstract

A semiconductor device comprises an active element and contacts that permit low-resistance external electrical connections. The active element includes an active layer formed from group II-VI elements, an n-doped layer on one side of the active it layer, and a p-doped layer on the other side of the active layer. The p-doped layer is a ZnSe-based alloy or a ZnTe-based alloy. There are electrical contacts to the n-doped layer and to the p-doped layer. The electrical contact to the p-doped layer includes a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer. The graded-alloy contact layer is a HgZnSSe-based graded-composition alloy where the p-doped layer is a ZnSe-based alloy, or a HgZnSeTe-based graded-composition alloy where the p-doped layer is a ZnTe-based alloy.

What is claimed is:  
     
       1. A device, comprising: 
       an active element including an active layer formed from group II-VI elements, an n-doped layer on one side of the active layer and a p-doped layer on the other side of the active layer, the p-doped layer having a composition selected from the group consisting of (1) a ZnSe-based alloy and (ii) a ZnTe-based alloy;  
       means for making electrical contact to the n-doped layer; and  
       means for making electrical contact to the p-doped layer, the means including a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer, the graded-alloy contact layer being formed from an alloy selected from the group consisting of (i) a HgZnSSe-based graded-composition alloy having a composition of Hg 1−x  Zn x  S 1−y  Se y  where the p-doped layer is a ZnSe-based alloy and (ii) a HgZnSeTe-based graded-composition alloy having a composition of Hg 1−x  Zn x  Se 1−y  Te y  where the p-doped layer is a ZnTe-based alloy, where, in each case, x and y each vary an amount within the range 0 to 1 inclusive between a location adjacent the p-doped layer and a location remote from the p-doped layer.  
     
     
       2. The device of claim  1 , wherein the n-doped layer comprises, 
       an n-type wide-bandgap cladding layer adjacent the active layer, and  
       an n+-type wide-bandgap contact layer adjacent the cladding layer.  
     
     
       3. The device of claim  1 , wherein the means for making electrical contact to the n-doped layer comprises a layer of a low work function metal. 
     
     
       4. The device of claim  3 , wherein the low work function metal is selected from the group consisting of indium, aluminum, cadmium, lead, and titanium. 
     
     
       5. The device of claim  1 , wherein the p-doped layer comprises 
       a p-type wide-bandgap cladding layer adjacent the active layer; and  
       a p-type wide-bandgap contact layer adjacent the cladding layer.  
     
     
       6. The device of claim  1 , wherein the means for making electrical contact to the p-doped layer further comprises 
       a layer of a high work function metal in contact with the graded alloy contact layer.  
     
     
       7. The device of claim  6 , wherein the high work function metal is selected from the group consisting of gold, platinum, nickel, palladium, and iridium. 
     
     
       8. The device of claim  1 , wherein the p-doped layer is a ZnSe-based alloy and the graded-alloy contact layer is a HgZnSSe-based graded-composition alloy. 
     
     
       9. The device of claim  1 , wherein the m p-doped layer is a ZnTe-based alloy and the graded-composition alloy. 
     
     
       10. The device of claim  1 , wherein the p-doped layer further includes at least one element selected from the group consisting of a Group II element, a Group VI element, and a Group VII element. 
     
     
       11. The device of claim  1 , wherein the active layer is photonically active. 
     
     
       12. A device, comprising: 
       an active element including an active layer formed from group II-VI elements, an n-doped layer on one side of the active layer and a p-doped layer on the other side of the active layer, the p-doped layer having a composition selected from the group consisting of a ZnSe-based alloy, and a ZnTe-based alloy;  
       means for making electrical contact to the n-doped layer; and  
       means for making electrical contact to the p-doped layer, the means including a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer, the graded-alloy contact layer being formed from an alloy selected from the group consisting of  
       a HgZnSSe-based graded-composition alloy having a composition of Hg 1−x  Zn x  S 1−y  Se y  where the p-doped layer is a ZnSe-based alloy, the graded-composition alloy having a composition of the ZnSe-based alloy adjacent to the p-doped layer and a composition selected from the group consisting of beta-HgS and HgSSe remote from the p-doped layer, and  
       a HgZnSeTe-based graded-composition alloy having a composition of Hg 1−x  Zn x  Se 1−y  Te y  where the p-doped layer is a ZnTe-based alloy, the graded-composition alloy having a composition of the ZnTe-based alloy adjacent to the p-doped layer and a composition selected from the group consisting of HgSe and HgSeTe remote from the p-doped layer, where, in each case, x and y each vary an amount within the range 0 to 1 inclusive between a location adjacent the p-doped layer and a location remote from the p-doped layer.  
     
     
       13. The device of claim  12 , wherein the doped layer is a ZnSe-based alloy. 
     
     
       14. The device of claim  13 , wherein the opposition of the graded-composition alloy as a function of distance from the p-doped layer is selected to maintain a substantially constant lattice parameter with decreasing bandgap in a first region extending away from the p-doped layer, and selected to decrease the bandgap without maintaining the lattice parameter constant in a second region extending further from the p-doped layer. 
     
     
       15. The device of claim  12 , wherein the p-doped layer is a ZnTe-based alloy. 
     
     
       16. The device of claim  15 , wherein the composition of the graded-composition alloy as a function of distance from the p-doped layer is selected to maintain a substantially constant lattice parameter with decreasing bandgap. 
     
     
       17. A device, comprising; 
       a p-doped layer having a composition selected from the group consisting of (i) a ZnSe-based alloy and (ii) a ZnTe-based alloy; and  
       means for making electrical contact to the p-doped layer, the means including a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer, the graded-alloy contact layer being formed from an alloy selected from the group consisting of (i) a HgZnSSe-based graded-composition alloy having a composition of Hg 1−x  Zn x  S 1−y  Se y  where the p-doped layer is a ZnSe-based alloy and (ii) a HgZnSeTe-based graded-composition alloy having a composition of Hg 1−x  Zn x  Se 1−y  Te y  where the p-doped layer is a ZnTe-based alloy where, in each case, x and y each vary an amount within the range 0 to 1 inclusive between a location adjacent the p-doped layer and a location remote from the p-doped layer.  
     
     
       18. An ohmic contact to a group II-VI p-type semiconductor, said ohmic contact comprising: 
       a first portion of a II-VI alloy having a first composition including a II-VI semimetal, said first portion formed on said group II-VI p-type semiconductor, and a second portion of II-VI material having a second composition including a II-VI semimetal, said second portion being spaced from said group II-VI p-type semiconductor by said first portion, wherein said second composition has a greater amount of II-VI semimetal than said first composition.  
     
     
       19. An ohmic contact to a group II-VI p-type semiconductor, said ohmic contact comprising: 
       a II-VI quaternary alloy including a II-VI semimetal in which the proportion of II-VI semimetal increases in a direction away from said group II-VI p-type semiconductor.  
     
     
       20. An ohmic contact to a group II-VI p-type semiconductor, said ohmic contact comprising: 
       a II-VI quaternary alloy having a first portion formed on said group II-VI p-type semiconductor and a second portion spaced from said group II-VI p-type semiconductor by said first portion, said II-VI quaternary alloy including a II-VI semimetal in which (1) the proportion of II-VI semimetal increases in a direction away from said group II-VI p-type semiconductor in said first and second portions and (2) the relative proportions of the group VI elements in the II-VI quaternary alloy are varied as the proportion of II-VI semimetal increases in order to maintain a substantially constant lattice parameter in said first portion.